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Implementing Security on Cisco Routers
Version 1.0g
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Route Tables and Routing Protocols
A router’s primary responsibility is to send a packet of data to the intended
destination. To accomplish this, each router needs a route table. Each router builds
its table based on information from the network and from the network administrators.
The router then uses a set of metrics, depending on the contents of the table and its
routing algorithm, to compare routes and to determine the ‘best’ path to a destination.
Routers use four primary mechanisms for building their route tables:
1. Direct connection: Any LAN segment to which the router is directly
connected is automatically added to the route table. For example, the
router Central is connected to the LAN segment 14.2.9.0/24.
2. Static routing. A network administrator can manually instruct a router to
use a given route to a particular destination. This method takes
precedence over any other method of routing.
3. Dynamic routing. Uses router update messages from other routers to
create routes. The routing algorithm associated with the particular
routing protocol determines the optimal path to a particular destinations,
and updates the route table. This method is the most flexible because it
can automatically adapt to changes in the network.
4. Default routing. Uses a manually entered route to a specific ‘gateway of
last resort’ when route is not known by any other routing mechanism.
This method is most useful for routers that serve as the sole connection
between a small LAN and a large network like the Internet. Routers that
depend on a single default gateway usually do not use routing protocols.
Although many different dynamic routing protocols exist, this section focuses on
only two: RIP and OSPF. These two are the most widely used standard routing
protocols. RIP, the Routing Information Protocol, is an example of a distance vector
based protocol. OSPF, or Open Shortest Path First, is an example of a link state
protocol. The table below provides a short comparison.
Table 4-2 – RIP v. OSPF
RIP
Distance vector protocol: maintains a list of the distances to other networks
measured in hops, the number of routers a packet has to traverse in order to
reach its destination. Limited in scale because any distance greater than 15 hops
is inaccessible. Broadcasts updates every 30 seconds to all neighboring RIP
routers to maintain integrity. Each update is a full route table.
OSPF
Link state protocol: uses a link speed-based metric to determine paths to other
networks. Each router maintains a simplified map of the entire network.
Updates are sent via multicast, and are sent only when the network
configuration changes. Each update only includes changes to the network.